Scott W. Nixon’s research while affiliated with University of Rhode Island and other places

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Publications (121)


Decomposition of mangrove litter under experimental nutrient loading in a fringe Rhizophora mangle (L.) forest
  • Article

September 2020

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46 Reads

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3 Citations

Estuarine Coastal and Shelf Science

Brita J. Jessen

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Candace A. Oviatt

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Ryann Rossi

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[...]

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Scott W. Nixon

Carbon (C) cycling is an important attribute of mangrove forests that relates to the structure, function, and resilience of mangroves under environmental change. Increased nutrient enrichment in tropical coastal waters may influence C cycling through organic C mineralization. For example, by alleviating nutrient limitation of the heterotrophic microbial community, nutrient enrichment may enhance C mineralization and facilitate a loss of within-stand C sequestration. Here, we enriched a coastal fringe Rhizophora mangle (L.) mangrove system for two years with two fertilizer regimes to mimic agriculture runoff (”+high” N:P ratio of 50:1) and urban runoff (”+moderate” N:P ratio of 16:1) scenarios as follows: (1) annual loading rate of 70 g N m ⁻² year ⁻¹ and 3.1 g P m ⁻² year ⁻¹ or (2) annual loading rate of 70 g N m ⁻² year ⁻¹ and 9.7 g P m ⁻² year ⁻¹. C mineralization was measured as microbial respiration rates from the forest floor and litter decomposition rates. While decomposing leaf litter and green leaves had lower molar C:N under the +moderate N:P fertilization course, neither fertilization scenario produced an effect on C mineralization processes compared with ambient conditions. Substrate CO 2 flux rates were not different among treatments and ranged from 1.15 to 1.81 μmol CO 2 m ⁻² s ⁻¹ (3.0–4.8 g CO 2 m ⁻² day ⁻¹) following 72 weeks of fertilization and 0.58–1.55 μmol CO 2 m ⁻² s ⁻¹ (1.5–4.1 g CO 2 m ⁻² day ⁻¹) 30 weeks following the end of the experiment. Time to 50% decay of above-ground leaf litter ranged from 61 to 110 days (average 79 days). Below-ground leaf litter material was fully decomposed by 22 months after burial. A¹⁵ N pulse-recovery suggests that the majority of the retained fertilizer (22.2 ± 4.4% at 10 months following spike) was taken up by fine roots, though this did not significantly affect CO 2 flux from the forest floor. This work demonstrates that nutrient enrichment by aqueous delivery does not strongly affect organic carbon mineralization in a coastal fringe mangrove within two years. Environmental conditions, substrate quality, and location may play a more substantial role in mangrove C dynamics compared with short-term aqueous-based nutrient enrichment.


A Reduced Complexity, Hybrid Empirical-Mechanistic Model of Eutrophication and Hypoxia in Shallow Marine Ecosystems

May 2017

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38 Reads

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3 Citations

Numerical simulation models have a long history as research tools for the study of coastal marine ecosystems, and are increasingly being used to inform management, particularly related to nutrient-fueled eutrophication. Demand for modeling assessments is rapidly increasing, and managers need generally applicable tools that can be rapidly applied with limited resources. Additionally, a variety of calls have been made for the development of reduced complexity models for use in parallel with more complex models. We propose a simplified, empirically constrained modeling approach that simulates the first-order processes involved in estuarine eutrophication, contains a small number of aggregated state variables and a reduced set of parameters, and combines traditional mechanistic formulations with robust, data-driven, empirical functions shown to apply across multiple systems. The model was applied to Greenwich Bay, RI (USA), a subestuary of Narragansett Bay, and reproduced the annual cycles of phytoplankton biomass, dissolved inorganic nutrients, and dissolved oxygen, events including phytoplankton blooms and development of hypoxia, and the rate of annual primary production. While the model was relatively robust to changes in parameter values and initial conditions, sensitivity analysis revealed the need for better constraint of the phytoplankton carbon-to-chlorophyll ratio, temperature dependence of phytoplankton production, and parameters associated with our formulations for water column respiration and the flux of phytoplankton carbon to the sediments. This reduced complexity, hybrid empirical-mechanistic approach provides a rapidly deployable modeling tool applicable to a wide variety of shallow estuarine systems.


Comparing Measures of Estuarine Ecosystem Production in a Temperate New England Estuary

June 2016

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101 Reads

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11 Citations

Estuaries and Coasts

Anthropogenic nutrient enrichments and concerted efforts at nutrient reductions, compounded with the influences of climate change, are likely changing the net ecosystem production (NEP) of our coastal systems. To quantify these changes, scientists monitor a range of physical, chemical, and biological parameters sampled at various frequencies. Water column chlorophyll concentrations are arguably the most commonly used indicator of net phytoplankton production, as well as a coarse indicator of NEP. We compared parameters that estimate production, including chlorophyll, across an experimental nutrient gradient and in situ in both well-mixed and stratified estuarine environments. Data from an experiment conducted in the early 1980s in mesocosms designed to replicate a well-mixed mid-Narragansett Bay (Rhode Island) water column were used to correlate changes in chlorophyll concentrations, pH, dissolved oxygen (O2), dissolved inorganic nitrogen, phosphate, and silicate concentrations, cell counts, and 14C carbon uptake measurements across a range of nutrient enrichments. The pH, O2, nutrient, and cell count measurements reflected seasonal cycles of spring blooms followed by late summer/early fall respiration periods across nutrient enrichments. Chlorophyll concentrations were more variable and rates of 14C productivity were inconsistent with observed trends in nutrient concentrations, pH, and O2 concentrations. Similar comparisons were made using data from a well-mixed lower Narragansett Bay station and a more stratified upper Narragansett Bay station in 2007 and 2008. Trends among pH, O2, and nutrient concentration parameters were similar to those observed in the mesocosm dataset, suggesting that continuous free water measurements of pH and O2 seem to reliably reflect ecosystem metabolism and, while not perfect measures, may be underused indicators of NEP.


Changes to nitrate isotopic composition of wastewater treatment effluent and rivers after upgrades to tertiary treatment in the Narragansett Bay watershed, RI

February 2016

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83 Reads

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14 Citations

Marine Pollution Bulletin

Due to nitrogen load reduction policies, wastewater treatment facilities (WWTFs) have upgraded to tertiary treatment — where denitrification reduces and removes nitrogen. Changes to the stable isotopic composition of nitrate inputs after upgrades or how it transfers to the estuary have not been assessed in Rhode Island. We investigate whether these upgrades impact the isotopic signature of nitrate inputs to Narragansett Bay. Samples from rivers and WWTFs discharging to Narragansett Bay characterize the anthropogenic source nitrate (NO3−) isotopic composition (δ15N–NO3− and δ18O–NO3−) and temporal variability. At one WWTF, tertiary treatment increased effluent nitrate δ15N–NO3− and δ18O–NO3− values by ~ 16‰. Riverine values increased by ~ 4‰, likely due to the combination of decreases in N and upgrades. Combined river and WWTF flux-weighted isotopic compositions showed enriched values and an amplitude reduction in monthly variability. When seasonal isotopic means are significantly different from other sources, δ15N–NO3− may be a useful tracer of inputs.


Fig. 1. (a) MODIS satellite mean summer (July to September) sea surface temperature image of Block Island Sound (BIS) and Rhode Island Sound (RIS) for 2002 to 2007. Note the warmer surface water in seasonally stratified RIS compared with the cooler, more well-mixed BIS. (b) Study area boundaries used for this study, shown with surface chl a and primary production sampling stations 
Fig. 2. Daily variable input values for the BZE models of primary production shown with in situ measurements. (a,c) Surface chl a was linearly interpolated (solid line) between daily average measurements for both Sounds. Points are the mean of triplicate samples collected at each station throughout the 22 mo sampling period. (b,d) Extinction coefficients (k, used to calculate the 1% light level) were extrapolated using the best fit 
Fig. 3. Annual (a,c) temperature and (b,d) salinity regimes at the primary production samplings stations in (a,b) Block Island Sound and (c,d) Rhode Island Sound. Black lines: days on which CTD casts were taken 
Fig. 4. Significant relationships between (a,b,c) chlorophyll and (d) primary production and measured forcing variables, including (a) depth of the euphotic zone, (b) water column stratification, (c) surface water salinity, and (d) surface water temperature. Regression equations represent data from both stations combined. BIS: Block Island Sound; RIS: Rhode Island Sound 
Fig. 5. Reduced major axis regressions of measured surface chl a versus SeaWiFs satellite values calculated using (a) the re gional algorithm developed by Pan et al. (2008, 2010) for the mid-Atlantic Bight, and (b) the traditional ocean OC4 algorithm for Rhode Island Sound (RIS), Block Island Sound (BIS), and samples collected outside of our study area (Offshore) 

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Comparison of surface chlorophyll, primary production, and satellite imagery in hydrographically different sounds off southern New England
  • Article
  • Full-text available

September 2015

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180 Reads

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2 Citations

Marine Ecology Progress Series

Block Island Sound (BIS) and Rhode Island Sound (RIS) are adjacent inner conti nental shelf ecosystems with contrasting hydrographic regimes. BIS exhibits more energetic tidal mixing, and water column stratification remains weak but persists year-round due to nearby estuarine exchange flow; RIS is less influenced by estuaries, and more seasonal with strong stratification in summer. We compared annual cycles of phytoplankton biomass and primary production in BIS and RIS using measurements (surface chlorophyll, C-14 primary production), primary pro duction models (Webb/Platt and BZE models), and satellite ocean color products. During 22 mo of sampling, measured surface chlorophyll was not significantly different between BIS (mean = 1.86 mg m(-3)) and RIS (1.69 mg m(-3)), and bimodal peaks of phytoplankton biomass and production occurred concurrently in both Sounds. In contrast, a 12 yr ocean-color based chlorophyll time series indicated higher long-term average surface chlorophyll in the more well-mixed system (BIS, mean = 1.50 mg m(-3); RIS, mean = 0.86 mg m-3). BIS annual primary production (318 to 329 g C m(-2) yr(-1)) was higher than RIS (239 to 256 g C m(-2) yr(-1); p < 0.001). These differences were most apparent during the summer, concurrent with the largest differences in water column stratification. Phytoplankton bloom phenology was driven by physical processes, with chlorophyll significantly related to water column stratification (r = -0.51, p = 0.01), depth of the euphotic zone (r = -0.54, p = 0.05), and surface water salinity (r = 0.54, p = 0.04). Primary production was correlated with surface water temperature (r = 0.57, p = 0.03) but the mechanisms underlying production differences between the Sounds remain unresolved. We hypothesize that different hydrographies give rise to different productivity between the Sounds.

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Fig. 2 Dissolved inorganic nitrogen (top panel) and dissolved silica (bottom panel) plotted with pH in the 2× mesocosm 
On the Response of pH to Inorganic Nutrient Enrichment in Well-Mixed Coastal Marine Waters

March 2014

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243 Reads

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28 Citations

Estuaries and Coasts

Recent concerns about declining pH in the surface ocean in response to anthropogenic increases of carbon diox-ide (CO 2) in the atmosphere have raised the question of how this declining baseline of oceanic pH might interact with the much larger diel and seasonal variations of pH in coastal marine ecosystems. Nutrient enrichment, which can amplify both production and respiration, has the potential to reduce or exacerbate the impacts of ocean acidification in coastal wa-ters. Here, we present results from a multi-year experiment in which replicate phytoplankton-based mesocosms with a 5-m deep well-mixed water column (salinity = 27–31) and intact benthic community were exposed to a gradient in daily inor-ganic nitrogen (N), phosphorous (P), and silica (Si) addition. We show that the response of water column pH to nutrient enrichment was the greatest during the autotrophic winter-spring period, and there was no significant decline in pH across treatments during the heterotrophic summer-fall period. We believe that the differences in response lie in the seasonal cycles of production and respiration, where spring production peaks are large and discrete, and respiration is more temperature-driven but occurs diffusely throughout the year. The observed basification associated with enhanced nutrient inputs may have consequences for phytoplankton community structure, some species of submersed aquatic vegetation, cy-cling of Si, and perhaps other ecological processes.


Table 1 . Examples excessive media headlines emphasizing the collapse of marine ecosystems due to ocean calamities. Headline Source Dates Plagues referred to 
Reconsidering Ocean Calamities

January 2014

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757 Reads

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71 Citations

BioScience

The proliferation of a number of pressures affecting the ocean is leading to a growing concern that the state of the ocean is compromised, which is driving society into pessimism. Ocean calamities are disruptive changes to ocean ecosystems that have profound impacts and that are widespread or global in scope. However, scrutiny of ocean calamities to ensure that they can be confidently attributed to human drivers, operate at widespread or global scales, and cause severe disruptions of marine social-ecosystems shows that some of the problems fail to meet these requirements or that the evidence is equivocal. A number of biases internal and external to the scientific community contribute to perpetuating the perception of ocean calamities in the absence of robust evidence. An organized auditing of ocean calamities may deliver a more precise diagnosis of the status of the oceans, which may help to identify the most pressing problems that need be addressed to conserve a healthy ocean.


Evidence and a conceptual model for the co-occurrence of nitrogen fixation and denitrification in heterotrophic marine sediments

May 2013

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147 Reads

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125 Citations

Marine Ecology Progress Series

Marine waters are often nitrogen (N) limited. Denitrification, the microbial conversion of nitrate to dinitrogen (N-2) gas, is responsible for significant N removal from the coastal ocean. In contrast, nitrogen fixation, the microbial transformation of N-2 to ammonium, is typically regarded as an inconsequential N source. The imbalance between these 2 processes is responsible, at least in part, for N limitation in the coastal ocean. Organic matter quality and quantity has been shown to determine rates of these critical N cycling processes. We hypothesized that the timing of organic matter deposition to the benthos might also be important in determining which process dominates. We tested this hypothesis using a coupled biogeochemical-molecular approach. We report directly measured net sediment denitrification rates and corresponding expression of nirS, a gene in the denitrification pathway, with the simultaneous expression of nifH, a gene associated with nitrogen fixation. The timing of organic matter deposition determined the magnitude of the net sediment N-2 fluxes. Highest rates of denitrification occurred soon after deposition, and the lowest rates occurred over 200 d after the last deposition event concomitant with increased nifH expression. Phylogenetic evidence suggests that sulfur and sulfate reducers are responsible for the nitrogen fixation. Globally, warming water temperatures, changes in light, and reduced nutrient loads through management intervention have been linked to decreases and/or altered phenology of water column productivity. Based on a conceptual model developed here, we suggest that in these systems, heterotrophic sediment nitrogen fixation may become an important component of the nitrogen budget.


Net sediment N2 fluxes in a southern New England estuary: Variations in space and time

November 2012

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106 Reads

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32 Citations

Biogeochemistry

Over the past three decades, Narragansett Bay has undergone various ecological changes, including significant decreases in water column chlorophyll a concentrations, benthic oxygen uptake, and benthic nutrient regeneration rates. To add to this portrait of change, we measured the net flux of N2 across the sediment–water interface over an annual cycle using the N2/Ar technique at seven sites in the bay for comparison with measurements made decades ago. Net denitrification rates ranged from about 10–90 μmol N2–N m−2 h−1 over the year. Denitrification rates were not significantly different among sites and had no clear correlation with temperature. Net nitrogen fixation (−5 to −650 μmol N2–N m−2 h−1) was measured at three sites and only observed in summer (June–August). Neither denitrification nor nitrogen fixation exhibited a consistent relationship with sediment oxygen demand or with fluxes of nitrite, nitrate, ammonium, total dissolved inorganic nitrogen, or dissolved inorganic phosphate across all stations. In contrast to the mid-bay historical site where denitrification rates have declined, denitrification rates in the Providence River Estuary have not changed significantly over the past 30 years.


Citations (98)


... Litter degradation starts with rapid leaching of dissolved organic (carbon, nitrogen, phosphorus, and tannin) and inorganic matter (Howard et al., 2020;Mamidala et al., 2022Mamidala et al., , 2023. Litter decomposition involves detritus feeding, microbial degradation and leaching of nutrients (Jessen et al., 2020, Shanij et al., 2023. During decomposition the chemistry of detritus changes completely (Van Vinh et al., 2020, Mamidala et al., 2023. ...

Reference:

Nutrient dynamics in mangrove ecosystem is a function of litter production and decomposition at sandspit backwater Karachi, Pakistan
Decomposition of mangrove litter under experimental nutrient loading in a fringe Rhizophora mangle (L.) forest
  • Citing Article
  • September 2020

Estuarine Coastal and Shelf Science

... Several of these models use mechanistic approaches incorporating physical forcing, nutrient cycling, food-web structure (limited to lower-trophic levels), as well as biological processes such as growth, predation, respiration, and decay (Kremer and Nixon 1978;Kremer and Kremer 1982;Nixon, Granger, and Nowicki 1995) in order to predict the population dynamics of plankton groups. Another approach that has proven effective in simplifying these Biological socioeconomic models is the use of empirically-based relationships to represent certain key processes (Brush et al. 2002;Brawley et al. 2003;Brush and Brawley 2009;Brush and Nixon 2017). This method integrates multiple processes into simplified bulk functions, resulting in a less complex model that is still robust in its prediction ability Nixon 2010, 2017;Borkman and Smayda 2016). ...

A Reduced Complexity, Hybrid Empirical-Mechanistic Model of Eutrophication and Hypoxia in Shallow Marine Ecosystems
  • Citing Chapter
  • May 2017

... These developments do not seem to have reduced the frequency and duration of hypoxia in the Providence River, as Mo concentrations in sediments from this time period remained elevated. It is likely that effluent initially discharged at Fields Point was essentially untreated sewage (Nixon et al., 2008) and the "treatment" did little to reduce the loading of nutrients and organic matter to the river waters. ...

Nitrogen and phosphorus inputs to Narragansett Bay: past, present and future
  • Citing Article
  • January 2008

... This study utilized direct comparisons between the calculated carbonate system parameters pCO 2 , pH T , and Ω A to DO%, the dynamics of which have been used as a proxy for estuarine productivity (Oczkowski et al., 2016;Oviatt et al., 1986), to provide additional insights into the variation of the carbonate system in Narragansett Bay in space and time. It is well-established that carbonate chemistry co-varies with oxygen and predictive models of carbonate chemistry in coastal systems have been developed based on in situ oxygen and environmental parameters such as temperature and salinity (Juranek et al., 2009;Alin et al., 2012;Reum et al., 2014). ...

Comparing Measures of Estuarine Ecosystem Production in a Temperate New England Estuary
  • Citing Article
  • June 2016

Estuaries and Coasts

... This may be due to our seeding method (DIS), where we obtained high seed densities by increasing the number of seeds per injected 'blob' (2 or 20 seeds blob -1 ) rather than increasing seed density by increasing the number of injections m -2 . This may actually have facilitated intraspecific competition rather than facilitation as was found by Granger et al., (2000). ...

The influence of planting depth and density on germination and development of Zostera marina seeds
  • Citing Article
  • January 2000

... Specifically, the consequences of eutrophication on eelgrass (Zostera marina) beds are well documented (Nixon and Pilson, 1983; Bowen and Valiela, 2001a; Bricker et al., 2007, 2008). Increased planktonic, epiphytic and benthic annual algae lead to decreased light penetration within the water column, direct shading and smothering from algal overgrowth, and increased consumption of oxygen at the sedimentewater interface from microbial decay of algal detritus. ...

Nutrient Cycling in Estuarine and Coastal Marine Ecosystems
  • Citing Chapter
  • January 1984

... This inference awaits support from additional evidence, including isotopic signatures and field investigations 28,58 . NO 3 − was not distinctly related to any other hydrogeochemical variables, invariably pointing towards the influence of widespread anthropogenic activities on the water quality [59][60][61] . The correlation coefficient between K + and Cl − in fracture groundwater (r = 0.84) was significantly higher than that in pore groundwater (r = 0.69). ...

Changes to nitrate isotopic composition of wastewater treatment effluent and rivers after upgrades to tertiary treatment in the Narragansett Bay watershed, RI
  • Citing Article
  • February 2016

Marine Pollution Bulletin

... Septic systems were identified as the major source of N entering most of the Salt Ponds as early as 1982 (Nixon et al. 1982;Lee and Olsen 1985). They continue to be the main source of N to these water bodies, and driving eutrophication (Nixon and Buckley 2007). ...

Nutrient inputs to Rhode Island coastal lagoons and salt ponds
  • Citing Article
  • January 1982

... In addition, the TON balance among watershed and atmospheric inputs, ocean export, and sediment burial would yield net DIN regeneration, which is proportional to respiration. Assuming fixed stoichiometry for both uptake and regeneration of DIN and TCO 2 , the difference between net uptake and net regeneration of DIN would provide a measure of P n ¼ P a À R n after converting DIN rates to equivalent C fluxes assuming fixed stoichiometry (e.g., Nixon and Pilson, 1984;Smith, 1991). ...

ESTUARINE TOTAL SYSTEM METABOLISM AND ORGANIC EXCHANGE CALCULATED FROM NUTRIENT RATIOS: AN EXAMPLE FROM NARRAGANSETT BAY
  • Citing Chapter
  • December 1984

... Although he was by no means the first to develop what is now referred to as literature meta-analysis, he convinced us single-handedly that it could be a powerful approach to discern large, fundamental patterns essential to understanding ecological linkages and processes (Nixon et al. 1986; Nixon 1980, 1982). He became interested in controls of coastal ecosystems, particularly nutrient enrichment, and pioneered use of large experimental mesocosms for water column (Nixon et al. 1984) and benthic studies (Taylor et al. 1999). Through all this, he demonstrated deep thought about concepts, made evident in his inspired simple and compelling definition of eutrophication (Nixon 1995), a slippery topic that most of us conceived less concisely. ...

Eutrophication of a Coastal Marine Ecosystem — An Experimental Study Using the Merl Microcosms
  • Citing Chapter
  • January 1984